On the shock compression of polycrystalline metals

At the present time, materials are being considered for use in increasingly extreme environments; extreme in terms of both the magnitude of the imposed pressures and stresses they encounter and the speed of the loading applied. Recent advances in understanding the continuum behaviour of condensed matter have been made using novel loading and ultrafast diagnostics. This insight has indicated that in the condensed phase, the response is driven by the defect population existing within the microstructure which drives plastic flow in compression as well as damage evolution and failure processes. This article discusses shock compression results, focusing upon research conducted on cubic-structured metals but also giving an overview of results on hexagonal-close-packed (HCP) metals and alloys. In the past, shock physics has treated materials as homogeneous continua and has represented the compressive behaviour of solids using an adaptation of solid mechanics. It is clear that the next generation of constitutive models must treat physical mechanisms operating at the micro- and mesoscale to adequately describe metals for applications under extreme environments. Derivation of such models requires idealized modes of loading which limits the range of hydrostatic or impact driven experimental techniques available to four principle groups. These are laser-induced plasma loading, Z pinch devices, compressed gas and powder-driven launchers and energetic drives and diamond anvil cells (DACs). Whilst each technique or device discussed brings unique advantages and core competencies, it will be shown that launchers are most capable of covering the spectrum of important and relevant mechanisms since only they can simultaneously access the material microstructural ‘bulk’ dimensions and timescales that control behaviour observed at the continuum. Shock experiments on a selection of metals whose response is regarded as typical are reviewed in this article, and sensors and techniques are described that allow the interpretation of the compression that results from idealized step loading on a target. Real-time imaging or X-ray techniques cannot at present access bulk states at the correct microstructural resolution, over a macroscopic volume or at rates that would reveal mechanisms occurring. It is controlled recovery experiments that provide the link between the microstructure and the continuum state that facilitates understanding of the effect of mesoscale properties upon state variables. Five metals are tracked through various shock-loading techniques which show the following characteristic deformation features; a low Peierls stress and easy slip allow FCC materials to develop dislocation cells and work-harden during the shock process, whereas the higher resistance to dislocation motion in BCC-structured materials and the lower symmetry in HCP metals slows the development of the microstructure and favours deformation twinning as an additional deformation mechanism to accommodate shock compression. Thus not only energy thresholds, but also operating kinetics, must be understood to classify the response of metals and alloys to extreme loading environments. Typical engineering materials possess a baseline microstructure but also a population of defects within their volumes. It is the understanding of these statistical physical relationships and their effects upon deformation mechanisms and defect storage processes that will drive the development of materials for use under extreme conditions in the future.